Gas generator charge with decreased temperature sensitivity

ABSTRACT

1. A gas generator charge comprising: A FIRST CYLINDRICAL SOLID PROPELLANT GRAIN HAVING DISPOSED CONCENTRICALLY THEREIN A SECOND LESS TEMPERATURE SENSITIVE SOLID PROPELLANT STRAND, WHEREIN SAID SECOND PROPELLANT STRAND IS SMALLER THAN SAID FIRST GRAIN SO AS NOT TO MATERIALLY CONTRIBUTE TO THE GAS PRODUCTION OF THE PROPELLANT AS WHOLE.

United States Patent 1 [111' 3,718,094

Bermender 1 Feb. 27, 1973 54] GAS GENERATOR CHARGE WITH 3,143,853 8/1964Sobey ..60/35.6 DEC REASED TEMPERATURE 2,681,619 6/1954 Chandler..60/225 SENSITIVITY Primary Examiner-Samuel Feinberg Inventor! NormanW-BBI'mQIIdQRWaCQTeX' Attorney-William R. Lane, Thomas S. MacDonald [73]Assignee: North American Aviation, Inc. and Stuart wohlgemuth Filed: J y1962 EXEMPLARY CLAIM [21] Appl. No.: 214,449 1. A gas generator chargecomprising:

a first cylindrical solid propellant grain having 60/254 disposedconcentrically therein a second less tempera- [51] Int. Cl ..F42b 1/00ture sensitive solid propellant strand, wherein said Field of Searchsecond propellant strand is smaller than said first grain 149/15, 16;102/49, 98, 100, 101 so as not to materially contribute to the gasproduction of the propellant as whole. [5 6] References Cited 0 '1Claim, 7 Drawing Figures UNITED STATES PATENTS 3,136,122 6/1964 McJones..102/98 X PAII-ZNTEnFmmu 1 saw 10F, 2 1

FIG. I

I/qI BHHSSHHd INVENTOR.

NORMAN W. BERMENDER AGENT GAS GENERATOR CHARGE WITII DECREASEDTEMPERATURE SENSITIVITY This invention concerns a novel solid propellantrocket motor. More specifically, the invention pertains to a solidpropellant rocket motor having reduced temperature sensitivity.

The use of a solid rocket motor as a gas generator for rocketapplications is well known. A gas generator is an assembly that willgenerate hot gas under pressure, for example, for supplying power to anycomponent or accessory of a rocket power plant. Typically, the gasgenerator is used mainly for driving the turbine of a turbo-pumpassembly, pressurizing propellant tanks, driving the turbine of anaccessory power supply or various other auxiliary power requirements ina rocket engine system. The hot pressurized gases delivered by the gasgenerator are a result of burning an internallycontained solid mixtureof an oxidizer and a fuel. The typical design of a solid generatorcomprises a combustion chamber which contains a solid reactant chargemade up of a solid mixture of a fuel and an oxidizer with probably otheradditives such as an inhibitor mixed therein. Additionally, there isincluded a means for igniting this solid reactant charge and there is adischarge outlet from the generator for attachment to v the system whichis to use the hot gases.

In operation the solid propellant charge is ignited by the flame of asquib igniter which is usually electrically excited. The charge burnsthus generating hot gases at very high pressure. These gases are emittedfrom the generator to an outlet into the system which utilizes thisenergy. The hot gases that are generated are not a pure medium in thatthey usually contain tiny particles of unburned propellant and residueof the combustion.

In gas generator applications it is particularly desirable and usuallynecessary that the solid propellant formation utilized be relativelycool-burning, that is, have a relatively low flame temperature. This canreadily be appreciated in view of the fact that the hot exhaust gasescome into contact with metal parts of turbines and the like. Because ofthis requirement, it is preferable that the propellant formulation usedbe basically of a propellant such as ammonium nitrate, which has thedesirable flame temperatures as well as velocity characteristics forsuch an application. However, when utilizing a cooler-buming propellantsuch as ammonium nitrate, a problem arises with regard to thetemperature sensitivity of the propellant. The temperature sensitivityis the sensitivity of a propellant to a change in the ambienttemperature which in turn results in an effect on the energy released aswell as the physical properties of the charge. The equilibriumtemperature of a solid propellant grain has a measurable effect on theburning rate and other properties obtained during the firing of thecharge. The initial temperature of the grain will also materially affectits performance. For example, on a hot day a given solid propellantrocket will operate at a higher chamber pressure and thrust than on acool day. The firing duration will be shorter but the total impulse willnot be changed significantly. This indicates that the initialtemperature of the grain has a deciding effect on the burning rate andthat weather conditions have to be considered when exacting performancerequirements are to be met. The temperature sensitivity for differentsolid propellants is usually expressed in percent per unit oftemperature change. Ordinarily the temperature sensitivity has valuesranging from 0.000 to 0.30 percentl F.

Since the initial temperature of storage affects the performance of therocket propellant grain especially as to burning rates and the amount ofthrust produced for a given duration of time, it is particularlydesirable to be able to make the propellant insensitive to the storagetemperature so that the propellant will burn at a given rate and chamberpressure regardless of temperature storage or in the case of rocketmotors the temperature of firing. Thus, dependable performance can beachieved at varying temperature conditions.

It is an object of this invention to effectively decrease thetemperature sensitivity of solid propellants.

It is a further object of this invention to provide a gas generatorwhich is relatively temperature insensitive.

Another object of this invention is to control the effective chamberpressure of a gas generator.

This invention accomplishes the above results in providing a lesstemperature sensitive propellant by utilizing a propellant grainconfiguration wherein the burning surface of the grain can be alteredduring buming by the use of a control strand of propellant. Thepropellant configuration of this invention comprises a thin strand ofcontrol propellant situated concentrically within the main gas-producingpropellant. The control strand of propellant is normally a lesstemperature sensitive propellant than the main charge propellant andprovides an insignificant contribution to the gas produced by thegenerator yet causes a pronounced change in the surface of thepropellant charge as a function of the propellant temperature.

It is believed that the invention will be better understood from thefollowing detailed description in which: v

FIG. 1 is a plot of the variation of chamber pressure with time for anine-pound cruciform grain at varying initial charge temperature.

FIGS. 2 and 3 are burning surface stability diagrams for two differentconfigurations of end-burning grains.

FIG. 4 is a diagram of a grain of this invention with a straight charge.

FIGS. 5a, 5b, and 5c are diagrams of the burning surface stability of aconed charge of this invention at three different charge temperatures.

As can be seen from FIG. 1 which is a diagram of the variation of thechamber pressure with time for a ninepound cruciform, semi-restrictedburning, plastic grain for 3.25-inch rocket motor, the propellant chargetemperature has a marked affect on the performance of the generator. Asis readily apparent from the diagram, the

higher the charge temperature, the shorter the burning duration andhigher the chamber pressure of the generator. Generally, it is desirableto have the generator performance at ambient or approximately Fconditions and it is the object of this invention to be able topre-program propellant charges so that the pressure-time curveapproaches that of ambient conditions or any particular conditionsdesired. It should be pointed out that the invention also has peculiarapplicability to rocket missiles carried by aircraft at high altitudesand low temperatures. As can be appreciated, there will be a markedaffect on such a missile as it goes from one extreme temperature toanother as it goes from air to ground.

Chamber pressure P is usually defined by the formula:

1 Spa G Wherein S equals the burning surface area of the grain in ft,

p is the density of the grain in lbs/fit equals a ballistic content forinitial grain temperature which varies between 0.002 and 0.05,

A, is the throat area of the nozzle used on the rocket chamber in ft andn is a ballistic constant which is between 0.0 and In the above formula,the ballistic constants a and n are varied depending upon the givenpropellant composition and are not set factors. As can readily be seenfrom the above equation, an increase in the burning surface area of thepropellant grain will effectively increase the chamber pressure of thegenerator. Thus, by so-increasing the burning surface one may raise thecurve as shown in FIG. 1 to a chamber pressure level as shown on the70-curve line by design. Alternatively, by decreasing the burningsurface the 150-curve may be brought down to correspond to the ambient70- curve shown in FIG 1. Previous to this invention one method ofchanging a chamber pressure and thus affecting the burning rate of thepropellant was to alter the size of the nozzle used to correspond todifferent temperatures of propellant. As can be seen from the formulagiven, the throat area of the nozzle also has a direct affect on thechamber pressure.

It is well established that in solid propellant burning the surfaceburned moves in the direction normal to itself at a uniform rate atevery point and burns in a radius about a given point. If there is anirregularity in the surface of the grain, by simple geometricconsiderations assuming burning in all directions, the surface willsmooth out and continue its regular recession as long as the desiredsurface is flat. As seen in FIG. 2, a cupshaped initially-surfaced grainwill eventually straighten out and the surface area be effectivelyreduced. This same phenomenon can be seen as illustrated in FIG. 3 wherea slot is made at the end of the grain to effectively increase theburning surface thereof. This particular slot will eventually disappearas well. FIGS. 2 and 3 were obtained from page 83 of Solid PropellantRockets by Clayton Huggett et a]., published by Princeton UniversityPress (1960). The same effect would be applied if one were to initiallyshape the surface of the grain in a conical configuration.

By placing a center strand of propellant in the gas generator having adifferent burning rate of that of the main charge, it has been foundthat the temperature sensitivity of the rocket motor can be greatlydecreased. More particularly, the center strand should be less sensitiveto temperature. For example, as seen in FIG. 4, a flat and burning grainhaving a center strand 11 less temperature sensitive propellant situatedtherein. It obtains a larger burning surface as the grain is burnedsince the control strand burns at a faster rate than that of the maincharge or propellant and creates, as can be seen, a conical-shapedsurface. This condition occurs when the grain is at a low temperatureand the center strand burns relatively faster. At station 12 'in FIG. 4both the main charge and control strand are in the same perpendicularplane at the end of the generator. At station 13 the less temperaturesensitive control strand M has burned at a relatively faster rate thanthe main charge 10 since the low temperature has not retarded theburning rate of the less temperature sensitive control strand ascompared to the more sensitive main charge. At stations 14 and 15, amore conical end surface is appearing due the previously discussedphenomena of the control strand burning faster than the main charge.Finally, at station 16, a completely conical end surface is obtainedgiven a much larger surface area and thus higher chamber pressure with aresulting increase in the overall burning rate of the whole propellantcharge. Obviously, equilibrium conditions are reached wherein a givenconical configuration will be maintained where the main charge andcontrol strand will be burning at a relatively same proportional rate.If desired, the equilibrium cone angle can be determined and the chargecan be pre-coned to this angle so that the same surface area can bemaintained throughout.

FIGS. 5a, 5b, and 5c represent the same initial conical-shaped grainwhich was burned at three different temperatures. In FIG. 5a the initialtemperature was As can be seen, the burning surface of the graingradually decreased. In FIG. 5b the initial temperature was 40 F and thesurface area of the grain remained approximately constant as designed,thus the chamber pressure remained fairly constant. In FIG. 5c theinitial temperature at which the grain was stored was 5 F and thesurface area of the main charge significantly increased as burningtranspired. In the three configurations shown in FIGS. 4, 5 and 6 thechamber pressure remained fairly constant at 400 psi.

Solid propellant temperature sensitivity is defined by the symbol 11-which is the temperature sensitivity of equilibrium temperature at aparticular value of K, where K is the ratio of the burning surface tothe throat area of the generator. 1r is normally expressed in percentlF. The lower the value of 77;; for a given propellant, the less itssensitivity to changes in temperature. Thus, in this invention alesstemperature sensitive propellant control center strand is used. That is,the center strand has a lower 1r, value than the main gas generatorcharge. As can be seen in FIG. 5a where the control strand is lesssensitive to changes in temperature, it will not burn relatively as fastas the outer charge and thus the conical shape will eventually disappearas shown. In FIG. 5b where slightly less than ambient conditions areoccurring, a conical shape is obtained rapidly and both the controlstrand and the main charge burn at the same rate maintaining the conicalshape. In FIG. 5c the less-sensitive center strand will burn at a fasterrate than the main charge at the 5 F temperature and thus the surfacearea of the grain is increased due to the effect of the center strandforcing the main charge into a conical burning shape. It has beendiscovered that through the use of the center control strand and thegeometry of the burning resulting therefrom, that the effective 11 of amotor will be that of the control propellant strand. Thus, it isdesirable to use a generally temperature insensitive control strand.

As can readily be seen, the generator can be designed to operate atdifferent conical angles depending upon the temperatures and propellantsused. If a unit is designed to operate at a conical angle about midwaybetween the design specification requirements for temperature range, thetime required for unit to change from one conical surface to anotherwill be minimized. For example, if a control angle of 45 is used at 70F, this angle might presumably decrease to approximately something like43 at a low temperature and increase to an angle something on the orderof 47 at a high temperature.

EXAMPLE I In order to demonstrate the invention, six end-burningpropellant charges were tested at three different temperatures, two eachat 40, 125 and 200 F. The base propellant was comprised of:

Weight Percent Butadienelmethylvinylpyridine 90/ 100 l2.06 Butylcarbitol formal 2.72 A mixture of 0.37N,N-diphenyl-para-phenylene-diamine and a complex diarylamine butanereaction product (this additive is termed Flexamine" and is made byUnited States Rubber Co., Naugatuck Chemical Division) Ferric ammoniumferricyanide 1.95 Magnesium oxide .49 Ammonium nitrate 80.00

The control strand was comprised of:

Carboxy terminated 14.73 linear polybutadiene (Butarez CTL made byPhillips Petroleum Co.) Butyl carbitol formal 2.94 Methyl aziridinylphosphine oxide .33 Ammonium perchlorate 64.00 Aluminum 16.00 Calciumoxalate 2.00

The control strand was 0.25 inch in diameter while the base charge was2.6 inches in diameter. The length of the base grain was 7.8 inches. Thethroat diameter of the nozzle was 0.115 inch in diameter. A pre-conedincluded angle of 60 was used for this particular combination ofpropellants based on ballistic calculations. The base propellant had aqr of 0.24; the control propellant had a rr of approximately 0.09.During the equilibrium portion of the traces obtained from the burningat the three temperatures, a rr value of 0.10 was obtained. This clearlyindicated that the units functioned in accordance with the theoryprescribed herein and that the effect of m, at equilibrium conditionsbecame that of the control strand of propellant. As a result, it canreadily be seen that the temperature sensitivity of a given propellantcan effectively be reduced by the use of the control strands of lesstemperature sensitive propellant as set forth herein.

The concept set forth in this invention is not limited to the propellantcompositions given in the example set forth. The problem that theinvention solves arises when a given propellant is chosen for usebecause of its desired ballistic characteristics and such propellant hasa relatively high temperature sensitivity. When this propellant ischosen, a control strand of a very temperature insensitive propellantcan be concentrically inserted, making the total charge as temperatureinsensitive as the control strand. As previously pointed out,

the control strand contributes virtually nothing to the ballisticcharacteristics to the charge since it is present in a relatively minutequantity. Thus, one is able to have a gas generator which is temperatureinsensitive yet possesses the desired ballistic characteristics. Inother words, temperature insensitivity is no longer a majorconsideration when choosing a given propellant for a gas generatorbecause of certain ballistic characteristics of the propellant.

From the previous results, it can be appreciated that the control strandof propellant having a lower temperature sensitivity effectivelycontrols the burning rate and burning surface area of the basepropellant charge used. At higher temperatures, the less-sensitivepropellant will burn relatively slower than the faster-buming basecharge and effectively retard and decrease the burning surface area soas to lower the chamber pressure to that obtained at designed ambientconditions. While alternatively, at lower temperatures theless-sensitive propellant strand will burn faster than the retarded basecharge and force the base charge into a larger burning surface area thusincreasing the chamber pressure to reach ambient condition.

The concept of this invention has applicability for virtually any typeof solid propellant compositions, the main requirement being that acenter control strand of temperature insensitive propellant be used.Thus, some of the types of propellants contemplated in the conventionaldouble base propellants which are usually comprised of mixtures ofnitrocellulose and nitroglycerin, solid propellants having ammoniumnitrate or ammonium perchlorate as the oxidizer with a particulate metalfuel and a binder of polybutadiene-acrylic acid copolymer (PFBB) orother forms of butadiene-type or polyurethanes, polysulfides,polyvinylchloride-type binder materials. Thus, it can be seen that thepropellant compositions per se are relevant to the invention only withregard to choosing one for its desired ballistic characteristics as themain propellant charge and choosing a control strand having a low 17,,value.

Since an infinite variety of solid propellants may be used, theinvention not being limited to any one particular type, the relativediameters of the control strand and main charge can be selecteddepending on the chamber pressure and burning rates desired. If it isparticularly desirous to pre-cone the propellant to the equilibrium coneangle, such can be done since the burning rates of propellants are knownand the calculations can be made to determine the cone that will beachieved at the operating temperature for the gas generator. Thediameter of the control strand is generally not critical and is usuallykept at a minimum size so that the control strand is relativelyinsignificant to the exhaust gases from the generator.

Additionally, it should be pointed out that a group of propellants havebeen developed which are virtually temperature insensitive at givenchamber pressure. These propellants are generally termed Mesapropellants. An example of such a propellant has the followingcomposition:

Weight Percent Nitrocellulose 50.0 Nitroglycerin 34.9 Diethyl hthalate10.5 2-ni phenylamine 2.0 Lead salicylate 1.2 Lead Z-ethylhexoate 1.2

Candelilla wax .2

This propellant is virtually temperature insensitive at approximately1,300 psi. Thus, if a gas generator was designed to operate at a chamberpressure of 1,300 psi and such a propellant were used as a controlstrand, the gas generator would be effectively temperature insensitiveregardless of the temperature sensitivity of the main charge utilized.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample only and is

1. A GAS GENERATOR CHARGE COMPRISING: A FIRST CYLINDRICAL SOLIDPROPELLANT GRAIN HAVING DISPOSED CONCENTRICALLY THEREIN A SECOND LESSTEMPERATURE SENSITIVE SOLID PROPELLANT STRAND, WHEREIN SAID SECONDPROPELLANT STRAND IS SMALLER THAN SAID FIRST GRAIN SO AS NOT TOMATERIALLY CONTRIBUTE TO THE GAS PRODUCTION OF THE PROPELLANT AS WHOLE.